[ On Friday, October 1, 1999 at 16:45:57 (-0500), Ron Roskens wrote: ]
> Subject: zs driver patch.
>
> The included patch updates the the zs driver, such that you need to key in
> a sequence ( BREAK, any char, BREAK, any char, BREAK ) before it will drop
> to the debugger/prom.

I'm not sure I like that idea any better -- sometimes I want to drop the
system into the debugger (or PROM) A.S.A.P. and without the possibility
that I'll make any mistake with fumbling fingers.  It's hard enough to
get the break sequence right with telnet/kermit/cu etc. in the first place!

The trick is to ensure that disconnecting the physical interface won't
cause the signal line to float in such a way that the UART detects it as
a BREAK.  It is said that a 4.7K-ohm resistor between pins 3 & 25 will
prevent a floating RxD from being seen as a BREAK signal (pin 25 on
systems that have separate A and B ports is supposedly a -5VDC source,
except for some versions [501-1899] of the 4/4xx VME CPUs [the A/B ports
on SLC, ELC, LX, Classic, SS10, etc. don't seem to have a -5vdc pin]).
However I've also heard people recommend using a resistor and zener
diode to keep the pull RxD line low.

Note that you'd have to mount the pull-down resistor *inside* the
chassis to make it fool-proof.

Note that the proper pull-down circuit shouldn't prevent normal data
pulses, or indeed a properly generated BREAK signal -- only the
accidental appearance of a BREAK due to a floating value.

WARNING:  though I've planned to do this myself many times (originally
on my Sun-3's), I've never actually done it!  ;-)

-- 
							Greg A. Woods

+1 416 218-0098      VE3TCP      <gwoods@acm.org>      <robohack!woods>
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